To avoid failure of wheel bearings in towed vehicles, lubrication of the wheel bearings must be maintained. For a typical wheel bearing used in a trailer, for example, grease is packed into the hub space surrounding the wheel bearing and weight-bearing axle. A cap is placed over the open-ended hub to keep the region free from dust and debris, and to keep the grease from migrating away from the bearing. Over time, the grease deteriorates and requires replacement or replenishment. The cap, wheel, bearing and so on may be removed periodically to allow repacking of the wheel-bearing grease so as to maintain proper operation. If such maintenance is not carried out, the loss of adequate lubrication may result in damage to, or seizing of, the wheel bearing.
To prevent such situations, a number of devices intended to keep wheel bearings lubricated have been developed, and are available in the marketplace. Some such devices seal the area around the wheel bearing and the hub to keep not only dust and debris away from the bearing, but also water and moisture. Other more advanced devices not only protect and shield the wheel bearing, but also provide enhanced lubrication. One such device is the bearing protector device disclosed in U.S. Pat. No. 6,199,662 to Felk. The device disclosed by Felk attaches to a wheel bearing housing and includes a spring-loaded piston. The device not only stops dirt and water from entering the hub, but also by means of a piston forces a reservoir of grease through a housing toward the wheel bearing. Although such known devices provide a degree of protection and lubrication, a user of such a device still may not easily determine whether additional lubrication need be added to the system.
However, several methods and devices are available to alert a user of a low-lubrication level. Some advanced vehicle bearing systems may employ sophisticated alarm systems to indicate low-lubrication levels. Other, less advanced systems, such as those used on lighter-load trailers, may rely on indirect, mechanical methods to alert a user to low-lubrication levels. One such device is a bearing-lubrication device as described in U.S. Pat. No. 4,941,550, and RE 34,391, both issued to Blake. The bearing lubricating devices of Blake attach to a wheel bearing housing and include a spring-loaded piston that forces grease through a housing of the device toward the wheel bearing, as is known in the art. The bearing lubricating device also includes an external mechanical indicator attached to the piston that indicates the piston position. Under ideal conditions, the position of the piston corresponds to an amount of lubrication in the device, such that a user may obtain an indirect indication of lubrication levels in the device.
Despite the availability of bearing lubrication and protection devices as described above, none provide direct, positive confirmation of actual lubrication levels within the device.